Firstly, welcome to the threads
I'm also new, but I'll do my best to help you. I'll answer your questions separately.
1) Yes, I believe. The scaling techniques should work with all files, regardless of origin.
2) That detached piece is probably floating in the same spot it is supposed to be positioned relative to your body. My recommendation would be to look in Halo 4 in the Theater, and determine where that piece ends relative to your anatomy, and then measure from the top of your shoulders, all the way down to where the piece ends. Looking here, I would guess it is positioned somewhere around the navel, right over your first 2 rows of abs. Remember to account for the raised shoulder features when you scale your model, though! They add some extra height.